Microfluidic devices are critical components for achieving miniaturization of a broad range of products including biochemical reactors, total chemical analysis systems, instrumentation, and systems-on-chip. Because the field has not yet matured, there is a need to quickly and inexpensively verify device designs. Thus, rapid prototyping techniques that can create complex, multi-functional microfluidic devices are sought by the art.
A wide variety of fabrication techniques have been described in the literature. M. J. Madou has provided a thorough overview in Fundamentals of Microfabrication: The Science of Miniaturization, 2nd ed. (CRC Press, New York, 2002). N.-T. Nguyen and S. T. Wereley have summarized fabrication techniques of particular utility in the construction of microfluidic devices in Fundamentals and Applications of Microfluidics (Artech House, Boston, 2002). To appreciate the advantages of the present invention it is useful to categorize fabrication processes by the manner in which pattern formation of the elements of a microfluidic device is effected.
A first category of fabrication approaches consists of those that rely on photolithography employing a fixed photolithographic mask to define the structural features of a microfluidic device. Included among this group are the well-known batch fabrication techniques applied in the semiconductor and microelectromechanical systems (MEMS) industries. Some particularly well-documented batch fabrication techniques used to build microfluidic devices include thick film photopatternable materials such as the epoxy material known as SU-8 and the class of microcontact printing techniques known as soft lithography. As is widely recognized in the art, batch fabrication techniques generally involve a very lengthy and complicated set of processing steps, including spin casting, material deposition, material etching, thermal processing, and surface cleaning and preparation.
In this first category of fabrication approaches, the fixed photolithographic mask may be used to directly expose a photopatternable material which upon development the remaining portions of the material form structural elements of a microfluidic device. This is generally the approach used in SU-8 fabrication. Alternatively, the fixed photolithographic mask may be used to expose a relatively thin photoresist layer which upon development forms a stencil that can be used to effect pattern transfer through subsequent process steps. This is generally the approach used in the semiconductor and MEMS industries where the developed photoresist layer forms an impermeable stencil against an etch process capable of removing the substrate material not protected by the stencil. After removal of the remaining photoresist the substrate exhibits a surface height modulation that corresponds to the pattern of the fixed photolithographic mask. Soft lithography uses this approach to create a microstamp or molding master in a hard substrate such as silicon. The molding master can then be used to transfer pattern features to a polymer such as polydimethylsiloxane which can be poured onto the molding master, cured in place, removed from the molding master, and adhered to a second substrate to form microfluidic channels.
A second category of fabrication techniques has been developed wherein the overall fabrication schemes of the first category of techniques are preserved with the one exception that lithographic exposure is performed by a programmable exposure unit without the need for a fixed photolithography mask. The pattern information required to form the elements of a microfluidic device is stored electronically as a computer file and thus can be quickly modified. Examples of programmable exposure units well known in the literature include rastered laser exposure systems and programmable two-dimensional arrays of optical modulators such as micromirrors.
Direct writing technologies form a third category of fabrication approaches. As with the programmable exposure units described above, pattern information is manipulated and stored electronically so that no fixed photolithography masks are required. However, instead of the generally lengthy and complicated fabrication schedules of the first two categories, direct writing technologies form structural elements of a microfluidic device directly on or in the substrate using few processing steps. Direct writing technologies can be either subtractive or additive processes. Laser machining, wherein substrate material is selectively removed under the influence of an intense optical beam, is a subtractive direct writing technology. Additive direct writing technologies include inkjet printing, pulsed laser ablation, microstereolithography, and micro-capillary deposition. The method described in the present invention uses an additive direct write technology employing micro-capillary deposition.
The fabrication approaches known within the current state-of-the-art all suffer from a large number of problems and limitations. The use of fixed photolithography masks imposes significant costs and time delays in the fabrication process. Substituting a programmable exposure unit for fixed photolithography masks involves costly capital equipment. Batch fabrication approaches generally involve a lengthy and complicated process schedule. These approaches also generally require large numbers of different types of expensive processing tools that carry high costs associated with floor space and environment, maintenance, and operations. Existing fabrication approaches all suffer a limited palette of materials, defined as the variety of materials a given processing tool can handle. For example, several of the techniques cited above are specific for polymers and have limited or no application to other materials such as composites, ceramics, or metals. Inkjet printing is limited to low viscosity liquids and cannot readily form thick film structural elements. Because of this limited palette of materials, devices formed from these fabrication approaches may be constrained to operate over restricted temperature ranges and may have inferior performance characteristics. Many of the current fabrication approaches impose severe limitations on substrate size, geometry, surface topography, and existing substrate features.
The present invention is directed to overcoming the deficiencies in the prior art.